An Al alloy thin wire, a very pure Cu alloy thin wire or an Au alloy thin wire have heretofore been used as materials for connecting electrodes on semiconductor elements to external leads. The Al alloy thin wire is bonded by a method comprising heating a lead frame in which semiconductor elements are arranged to about 150.degree. to 300.degree. C., and using ultrasonic press bonding. Since Al is extremely oxidizable, ball bonding is generally not practiced. The very pure Cu alloy thin wire or Au alloy thin wire are generally bonded by the following procedures: the tip portion of the thin wire is heated and melted with an electric torch to form a complete sphere through surface tension; the ball portion is press bonded to the electrode on a semiconductor element which is heated to 150.degree. to 300.degree. C.; the thin wire on the lead side is further bonded to the lead, which has been plated with Ag or Pd, by ultrasonic press bonding. However, there arise problems as described below in the case of the highly pure Cu alloy thin wire. Cu reacts with the oxygen in the air during ball formation, and the ball surface is oxidized. In addition, the oxygen content of the ball portion becomes high, and consequently the ball shape becomes unstable. Moreover, since the hardness of the ball increases with an increase in oxygen, damage is caused to the part of Si chip under the electrode at the time of wire bonding. Accordingly, blowing inert gas is necessary for preventing the oxidation. As a result, Au has been mainly used for an alloy thin wire which is then capable of forming a stabilized complete sphere ball in air as a connecting material and which has a low electric resistance and a good thermal conductivity.
As the result of the development of fine processing techniques for semiconductor elements, such as LSIs, the density thereof has increased. Since the amount of Au alloy thin wires used in the semiconductor devices has increased, the supply of a new wire bonding material which is prepared at low cost and is excellent in reliability is in demand.
A material for wire bonding which is to replace the Au alloy thin wire is required to satisfy the following conditions: the material cost is less than Au, a ball can be stably formed, an appropriate loop shape can be obtained, the strength of the bonded portion and the reliability of the bonded portion subsequent to resin sealing are at least equivalent to those of the Au alloy thin wire, and no electromigration takes place.
Pd may be used as a metal material satisfying the conditions mentioned above. Japanese Unexamined Patent Publication (Kokai) Nos 59-177339 and 59-201454 have proposed to use Pd as a material for wire bonding. Japanese Unexamined Patent Publication (Kokai) No. 59-177339 has proposed a bonding wire comprising one or more rare earth elements in an amount of 0.0005 to 2% by weight and optionally one or more of Ge, Bc and Ca in an amount of 0.5%. Japanese Unexamined Patent Publication (Kokai) No. 59-201454 has disclosed Pd defined to have a purity of at least 99.995%, contents of Pt and Fe each less than 15 ppm and a Vickers hardness of less than 60 at the ball portion. As the result of examining the literature, including the patent publication mentioned above, it has become evident that a Pd alloy thin wire has disadvantages as described below. Even Pd having a purity of at least 99.995% exhibits a low loop shape compared with a Au alloy thin wire. Pd containing rare earth elements exhibits a further lower loop shape, and the edge of the semiconductor element may be contacted by the Pd thin wire during bonding. Since the ball becomes hard, the semiconductor element then suffers chip damage immediately under the electrode when the semiconductor element has no passivation film having a sufficient thickness under the electrode and the reliability of the semiconductor device may sometimes be impaired. Furthermore, it has become evident that when the Pd thin wire is ultrasonically press bonded to an inner lead of the lead frame at the time of second bonding, the wire may sometimes be insufficiently bonded.